Final answer:
The Apgar test, with high combined scores on criteria such as skin color, heart rate, reflex, muscle tone, and respiration, best indicates a successful transition for a 38-week gestation newborn after birth.
Step-by-step explanation:
The best indicator that a 38-week gestation newborn infant is transitioning well to extrauterine life immediately following a vaginal birth is a high score on the Apgar test. This simple assessment evaluates five criteria: skin color, heart rate, reflex, muscle tone, and respiration. Each criterion is assigned a score of 0, 1, or 2 based on the infant's condition at 1 minute and again at 5 minutes after birth. The summation of these scores, with a maximum score of 10, reflects the newborn's overall health and ability to adapt to life outside the womb. High scores indicate a successful transition, while lower scores may signal distress and the need for medical intervention.